;;
;; Examples using abstract list functions
;;

;; --------------------------- map --------------------------------

;; add1*: (listof Number) -> (listof Number)
;; adds 1 to all numbers in l
(define (add1* l)
  (cond
    [(empty? l) empty]
    [else (cons (add1 (first l)) (add1* (rest l)))]))

;; Examples/Tests
(empty? (add1* empty))
(equal? (add1* (list 1 2 3)) (list 2 3 4))

;; add1*/map: (listof Number) -> (listof Number)
;; adds 1 to all numbers in l
(define (add1*/map l)
  (map add1 l))

;; Examples/Tests
(empty? (add1*/map empty))
(equal? (add1*/map (list 1 2 3)) (list 2 3 4))


;; --------------------------- filter --------------------------------

;; negs: (listof Number) -> (listof Number)
;; produces a list of on only the negative numbers in l
(define (negs l)
  (cond
    [(empty? l) empty]
    [else (cond
            [(negative? (first l)) (cons (first l) (negs (rest l)))]
            [else (negs (rest l))])]))

;; Examples/Test
(empty? (negs empty))
(equal? (negs (list 1 -1)) (list -1))

;; negs/filter: (listof Number) -> (listof Number)
;; produces a list of on only the negative numbers in l
(define (negs/filter l)
  (filter negative? l))

;; Examples/Test
(empty? (negs/filter empty))
(equal? (negs/filter (list 1 -1)) (list -1))


;; --------------------------- foldr --------------------------------

;; sum: (listof Number) -> Number
;; sums the numbers is l
(define (sum l)
  (cond
    [(empty? l) 0]
    [else (+ (first l) (sum (rest l)))]))

;; Examples/Tests
(zero? (sum empty))
(= (sum (list 1 2 3)) 6)


;; sum/foldr: (listof Number) -> Number
;; sums the numbers is l
(define (sum/foldr l)
  (foldr + 0 l))

;; Examples/Tests
(zero? (sum/foldr empty))
(= (sum/foldr (list 1 2 3)) 6